It has been recognized that the permittivity of certain (generally homogeneous) media varies with changes in the homogeneity of the media. Generally, an increase (or decrease) in the content of a sample of first material relative to a sample of a second material; wherein the permittivity of the first material is sufficiently different compared to the permittivity of the second material, will generally cause a change (either an increase or a decrease) in the permittivity of the homogeneous (or near homogeneous) media formed by both samples uniformly mixed.
In the case of water and soil, a variety of devices have been developed to sense the moisture content of soil. These devices can be used, for example, to control irrigation or drainage systems. In such devices, a sample of soil having a predetermined volume is monitored and its moisture content is sensed. The moisture content of the soil surrounding the sample is assumed to be representative of the moisture content of the soil sample itself. It is important, therefore, that the information provided by the devices is based solely on the moisture content of the soil rather than variations in some other parameters which are specific to the soil sample but not to the surrounding soil.
Conventional sensors have typically included a pair of parallel plates which surround a soil sample. The combination of the parallel plates and soil sample form a variable capacitor that is coupled to an electronic circuit. By being positioned between the parallel plates, the soil operates as the capacitor's dielectric. Since variations in the soil's moisture content causes variations in its permittivity, the value of the capacitor changes with variations in the soil's moisture content.
In order to take advantage of this known phenomenon, some conventional soil moisture sensors have incorporated the aforementioned variable capacitors into either resistor-capacitor ("RC") or inductor-capacitor ("LC") oscillator circuits. As is well known, the oscillation frequencies of RC and LC circuits vary with the capacitance value of the capacitor if all other parameters are held constant. Hence, some have thought that they can accurately determine the moisture content of a soil sample by detecting the oscillation frequencies of an RC or LC circuit where the capacitor of the circuit incorporates a pair of parallel plates which sandwich the soil sample therebetween. Prior RC and LC circuits, however, have not been able to consistently measure the oscillation frequency attributable to the moisture content of the medium being monitored for several reasons.
For example, in the case of RC circuits, variations in the oscillation frequency of RC circuits are not only attributable to the permittivity of the sample but are also attributable to the sample's conductivity. The sample's conductivity can be drastically affected by the soil's salinity, which can vary greatly over relatively small distances. Furthermore, the amount of the soil's conductive components in a sample, which can also vary greatly over relatively small distances due to its generally non-homogeneous nature, can also affect its conductivity.
Because of these non-uniformities, it is nearly impossible to factor out the effects of the soil's conductivity from the effects of the soil's permittivity. Accordingly, RC circuits are not particularly well-suited for measuring the moisture content of soil.
Due to the problems associated with RC oscillator circuits, there has been a recent trend to replace them with LC oscillator circuits, whose resonance frequency is relatively independent of the conductivity of its dielectric, when sensing the moisture content of soil. U.S. Pat. No. 5,445,178 to Feuer, for example, discloses a moisture sensor which employs an LC oscillator circuit. The sensor includes a pair of elongated sensor elements which have a rectangular cross-section, are spaced apart by a gap and are coupled to an electronic module. A pair of braces are used to maintain a uniform gap width along the length of the sensor elements. The sensor elements, which function as capacitor plates, are installed in soil by pushing or burying them so that the soil is between and around the sensor elements. The soil, therefore, functions as the dielectric of the capacitor.
As is well-known, U.S. Pat. No. 5,445,178 discloses that the LC oscillator defines a resonance frequency f.sub.o dependent upon the value L of the inductor and the value C of the capacitor based generally on the equation: ##EQU1##
By choosing an appropriate inductance value, one can set the frequency range of an LC oscillator. In the preferred embodiments of U.S. Pat. No. 5,445,178, the inductance value was chosen so that the LC oscillator circuit operates in the frequency range of approximately 10 kHz to 10 MHZ. Oscillators which operate in that frequency range, however, suffer from problems, in that, the resonance frequency of the LC circuit is not properly indicative of the moisture content of the soil sample.
Specifically, it has been found that, at low frequencies, capacitors comprised of dielectrics having characteristics similar to soil are subjected to the Maxwell-Wagner effect (also known as interfacial polarization), whereby charges having a polarity opposite to the polarity of the capacitor's plates "line-up" along the plates. In an LC oscillator circuit, the polarity of the capacitor's plates varies with its oscillation frequency. At low oscillation frequencies, the above-described charges can respond quickly enough to "line-up" along the plates. Consequently, as will be understood by those skilled in the art, the resulting frequency of the system is such that the calculated permittivity of the dielectric appears different from its actual permittivity.
It has been found that at frequencies higher than 27 MHZ, the charges cannot follow the polarization changes in the capacitor's plates. Therefore, capacitors operating at such frequencies are not subjected to the Maxwell-Wagner effect. Because the preferred embodiment of the device disclosed in U.S. Pat. No. 5,445,178 operates at frequencies substantially less than 27 MHZ, it is believed that the device suffers from the Maxwell-Wagner effect and is incapable of delivering accurate results consistently.
The device disclosed in U.S. Pat. No. 5,445,178 also suffers from a number of other deficiencies. First, because the sensor elements are metallic and exposed (i.e., it has no insulating dielectric sheath), the device is temperature dependent. Second, both the sensor elements and the spacers must be specially machined and, therefore, are relatively costly. Third, because the spacers must properly fit the sensor elements, both the sensor elements and the spacers must be manufactured according to relatively tight tolerances. Fourth, the sensor elements, because they are made of metal, are susceptible to corrosion and decomposition when placed in acidic mediums. Fifth, because of the surface area spanned by the plates and their lack of flexibility, the device is relatively difficult to place in the medium. Finally, the combination of the mechanical and electrical components described in U.S. Pat. No. 5,445,178 yields a device which is relatively complex and expensive.
Another patent which discloses an LC oscillator is U.S. Pat. No. 5,418,466 to Watson et al. U.S. Patent No. 5,418,466 is directed to a moisture and salinity sensor. The device includes a support frame for accommodating an array of sensors located within an access tube which has been inserted into a prepared hole in the soil. The sensors include a capacitive element in the form of upper and lower conductive rings which are disposed in spaced relationship to each other to maintain a constant air gap. The LC oscillator circuit changes its resonance frequency in response to moisture changes in the soil under the capacitive element's "sphere of influence."
Although the device of U.S. Pat. No. 5,418,466 is designed to operate at frequencies above 27 MHZ, like the device disclosed in U.S. Pat. No. 5,445,178, the device of U.S. Pat. No. 5,418,466 suffers from a number of deficiencies. First, the sensor elements, access tube, support and conductive rings must be specially machined. Second, because all of the aforementioned elements must fit together, they must be made under relatively tight tolerances. Third, devices of this kind seem to suffer from mechanical stress and shock during installation. The resulting device, including its mechanical and electrical components, therefore, is relatively complex and expensive.
Accordingly, there is a need for a permittivity sensor which overcomes many, if not all, of the aforementioned deficiencies of the prior systems.
It would be advantageous to provide a permittivity sensor which includes one or more of the following advantages: (1) relatively inexpensive and not complex; (2) not subject to relatively tight tolerances; (3) capable of being manufactured using "off-the-shelf" products; (4) not temperature dependent; (5) resistant to acidic mediums; (6) tolerant to mechanical shock; and, (7) easy to place in a monitored medium.